Invasive pneumococcal disease among HIV-1 seropositive individuals in the era of highly active antiretroviral therapy: does HIV-1 impair the macrophage host response to pneumococci?

Abstract

BackgroundIn the era of highly active antiretroviral therapy, incidence of bacterial pneumonia and invasive pneumococcal disease among HIV-1 seropositive individuals is still more than 30 times higher than for seronegative individuals. Risk remains elevated in those with CD4 cell counts in the normal range. A programme of host-mediated macrophage (Mϕ) apoptosis ensures killing of Streptococcus pneumoniae when canonical phagolysosomal killing capacity is exhausted. HIV-1 infection is associated with resistance of Mϕ to apoptosis. We hypothesised that HIV-1-mediated Mϕ apoptotic resistance impairs S pneumoniae killing. MethodsThe following Mϕ models were used: healthy donor monocyte derived Mϕ (MDM) infected with HIV-1BaL (HMDM) or sham infected (SMDM), with infection confirmed with anti-p24 immunohistochemistry; and MDM treated with gp120 10–100 ng/mL (gpMDM) or vehicle (cMDM). Mϕ were challenged with opsonised type 2 S pneumonia multiplicity of infection=10 or mock-infected and apoptosis up to 20 h by observing morphological changes (counting condensed or fragmented nuclei), counting rates of positivity with the TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labelling) assay, measuring cells with hypodiploid DNA, and measuring caspase-3/7 activity. Bacterial internalisation and killing were measured by gentamicin protection assay. Data are mean and SD, and analysed by paired t test, or are median and IQR with Wilcoxon matched pairs signed rank test or Kruskal-Wallis test if non parametric. FindingsAfter S pneumoniae challenge, rates of initial S pneumoniae internalisation were similar for all MDM conditions. HMDM showed fewer apoptotic nuclei than did SMDM (19% [IQR 18–21] vs 33% [26–43], n=7, p<0·05), and smaller increases in caspase-3/7 activity and higher Mcl-1 expression on western blot after S pneumoniae challenge. Compared with cMDM, fewer gpMDM developed apoptotic nuclei after S pneumoniae challenge, which was dose dependent (cMDM 36% [IQR 30–48] vs gpMDM 20% [12–28] at 10 ng/mL and 18% [14–22] at 100 ng/mL, n=12, p<0·01, confirmed with TUNEL), and gpMDM exhibited significantly less caspase 3/7 activity (p<0·05). In gpMDM, reduced apoptosis was associated with bacterial survival. InterpretationHIV-1 infection reduces Mϕ apoptosis in response to S pneumoniae, and gp120 may be sufficient to mediate this. Apoptosis resistance may impair killing of S pneumoniae after phagocytosis and increase susceptibility to invasive pneumococcal disease in HIV-1 seropositive individuals. FundingUK Medical Research Council.